1. Field of the Invention
The present invention relates generally to the field of electrochemical detection, and is more particularly directed to a series of novel chalcogenide glass compositions that may be used as sensor materials for the selective detection of metallic ions in solution.
2. Description of the Related Art
With the advent of ion selective electrodes (also known as ISEs), the use of electrochemical detection in industrial and medical applications has virtually exploded over the last few decades. ISEs provide a viable and convenient means for measuring the concentration of a specific ionic species in solution. In so doing, flowing samples can be continuously monitored such as to insure water quality, to identify contaminant levels in industrial effluent, and to attain optimum yields in chemical processes. Aside from the more conventional ISEs having an internal reference solution and electrode, solid state devices such as all solid state ISEs which are somewhat more durable are being developed. In addition, ion selective field effect transistors (ISFETs) are known which are so compact as to potentially enable implantation in patients, for example, to monitor critical ionic levels during surgery and the like.
The sensor materials utilized in ISEs and ISFETs to construct the sensing element or selective membrane of the device, generally need exhibit ionic conductivity and be selective or responsive to a specific ionic species. Known sensor materials include crystalline electrolytes such as pressed pellets of silver sulfide by itself or in combination with silver halides or other sulfide salts. See e.g. U.S. Pat. No. 3,591,464 to Frant. While these crystalline materials have utility, their overall performance in terms of selectivity can be altered in the presence of corrosive media such as acids and in the presence of strong oxidizing agents. In addition, the crystalline structure may be less durable in certain environments so as to detract from potential stability at the crystalline membrane surface over time. For these reasons, particularly in the case of solid state ISEs and ISFETs, there is a need to improve sensor materials known in the art to provide a stable device in varying environments and over long periods of usage.
Another type of sensor material known in the field is chalcogenide glass. Chalcogens, the elements of the sixth group of the Periodic Table including sulfur, selenium and tellurium as well as their binary, ternary and multi-component compounds with the elements of the third, fourth and firth groups such as germanium, phosphorous and antimony, can be readily obtained as amorphous vitreous solids. Chalcogenide glasses are promising membrane materials, particularly for solid-state ISEs and ISFETs, because they are generally more chemically stable in comparison with the corresponding crystalline materials.
Chalcogenide glass membranes comprising 60% selenium (Se), 28% germanium (Ge) and 12% antimony (Sb), and doped with elemental iron, cobalt and/or nickel were early on developed showing a selective response to iron (Fe.sup.+3) and copper (Cu.sup.+2) ions. C. T. Baker and I. Trachtenberg, J. Electrochem. Soc., 118. 329 (1971). Other vitreous chalcogenide compositions have since been disclosed which exhibit sensitivity to other metallic ions in solution including copper, lead, cadmium and silver for example. See, Yu. G. Vlasov and E. A. Bychkov, Ion-Selective Electrode Rev., 1987, Vol. 9, pp. 5-89 (made a part hereof and incorporated herein by reference). These chalcogenide membranes showed little change in selectivity even in the presence of strong acids and oxidation agents. Furthermore, the electric properties of these amorphous solids appear uneffected by occasional impurities which could otherwise negatively impact consistency of results.
In conjunction with their investigations respecting chalcogenide electrode materials, the present inventors observed that certain chalcogenide glass membranes develop a modified surface layer on the membrane surface upon contact with a test ionic solution. This modified layer is characterized by a considerable concentration of exchange sites for ion migration from the solution to the exchange sites and vice versa. It is believed that the direct ion exchange between solution and this modified surface layer is a potential-generating process that is extremely stable. Thus, the inventors next area of inquiry resided in developing specific chalcogenide formulations that would include this modified surface having a high concentration of ionic exchange sites, and further have the ability to affect a gradual transition from ionic conductivity at the membrane/solution interface to electronic conductivity for measurement. Excessive electronic conductivity would detract from the selectivity of the electrode, while too low a value of electronic conductivity would hinder its use in all solid state and ISFET devices. In combination with those attributes, the inventors sought specific chalcogenide compositions that exhibited high selectivity for specific ions in solution, even higher than conventional crystalline materials, so as to provide not only a more durable and stable electrochemical device but to provide a more sensitive device as well.
Thus, it is a primary object of the present invention to provide a chalcogenide glass sensor material for measuring the presence of ions in solution that is relatively highly selective for a specific ion in solution.
It is another object of the present invention to provide an improved chalcogenide glass sensor material that is durable and effective over extended periods of in-use service.
A further object of the present invention is to provide a chalcogenide glass sensor material that exhibits relatively high precision, stability and reproducibility of measurement.
It is also an object of the present invention to provide a chalcogenide glass sensor material that is relatively stable in the presence of oxidizing agents and corrosive media such as strongly acidic media.
Another object of the present invention is to provide a chalcogenide glass sensor material that exhibits both ionic conductivity and electronic conductivity in such a manner as to affect a gradual transition of ionic conductivity to electronic conductivity for measurement.
Yet another object of the present invention is to provide a chalcogenide glass sensor material that is particularly suitable for use in an all solid state ISEs and ISFETs constructed to achieve all the objects heretofore set forth.
A further object of the present invention is to provide a novel chalcogenide glass sensor material for the selective detection of cadmium.
Another object of the present invention is to provide a novel chalcogenide glass sensor material for the selective detection of copper.
Yet a another object of the present invention is to provide a novel chalcogenide glass sensor material for the selective detection of silver.
Still a further object of the present invention is to provide a novel chalcogenide glass sensor material for the selective detection of lead.
It is another object of the present invention to provide a novel chalcogenide glass sensor material for the selective detecting of thallium.
It is yet a further object of the present invention to provide a novel chalcogenide glass sensor material for the selective detection of mercury.
Still another object of the present invention is to provide an electrochemical device for the selective detection of metallic ions in solution comprising a novel chalcogenide glass sensor material as heretofore described.